Candiru fish.

Why they're dangerous: Perhaps you've heard of these dreaded, pinkie-size parasites, which live in the Amazon. They're the fish that...well, we'll let Wade explain. "What you hear is that they swim up the urine stream if you're [peeing] in the water," he says. "Basically, it will swim up the urethra. They have backward-facing spines on their gill flaps, so once they've got their head in, you can't get them out. They'll just wriggle up. I spoke to someone who this happened to, and he said he was hanging onto the tail, but it was so slippery it just disappeared."

How to stay safe: Should you find yourself swimming in the Amazon and are taken with a sudden urge to urinate, take a moment to step out of the water first. Seriously. "If it does happen to you, the only answer is surgery," says Wade. "Get yourself to a doctor quickly."